Since a flame-resistant fiber is excellent in heat resistance and flame resistance, it is widely utilized, for example, in a spatter sheet for protecting a human body from high-heat iron powder flying at welding work, or a welding spark and, further, in a flameproof heat insulating material of an aircraft, and a demand for the flame-resistant fiber in those fields is increasing.
In addition, the flame-resistant fiber is also important as an intermediate raw material for obtaining a carbon fiber. The carbon fiber is widely used in various utilities, for example, aviation/space aeronautical materials such as aircrafts and rockets, and sports goods such as tennis rackets, golf shafts and fishing rods and further, is going to be used in transportation machinery utility fields of ships and automobiles, because of excellent dynamical properties, various chemical properties and lightness. Furthermore, in recent years, application to electronic instrument parts such as cases of mobile phones and personal computers, and electrode utilities of fuel cells is strongly demanded, because of high electrical conductivity and heat radiation properties of the carbon fiber.
The carbon fiber is generally obtained by heating a flame-resistant fiber at a high temperature in an inert gas such as nitrogen, and carbonization-treating the fiber. Further, a conventional flame-resistant fiber, for example, a polyacrylonitrile (PAN) flame-resistant fiber is obtained by subjecting a PAN precursor fiber to a flame-resisting reaction (cyclization reaction+oxidation reaction of PAN) at a high temperature of 200 to 300° C. in an air. This flame-resisting reaction is an exothermal reaction, and is a reaction in a fiber form, that is, in a solid phase state. For this reason, long time treatment is necessary for temperature control and, to complete flame-resisting in a desired time, it is necessary to limit a single fiber fineness of the PAN precursor fiber to a small fineness of a specified value or less. Thus, it cannot be said that the currently known flame-resisting process is a sufficiently effective process.
As one method of solving the aforementioned technical problems, solutionization with a solvent was being studied. For example, the technique of heat-treating an acrylonitrile polymer powder in an inert atmosphere until the density is 1.20 g/cm3 or more, dissolving it in a solvent to fiberize it, and heat-treating the resulting fibrous material has been proposed (see JP-B No. 63-14093). However, since this proposal uses an acrylonitrile polymer powder, a flame-resisting reaction of which has not proceeded, there is a problem that change in a viscosity of the solution with time is great, and yarn breaking easily occurs frequently. In addition, since as a solvent, a strongly acidic solvent such as sulfuric acid and nitric acid which easily discomposes a general organic polymer is used, it is necessary to use an apparatus of a special material having anti-corrosion, being not practical from a view point of a cost.
In addition, a method of mixing a heat-treated acrylonitrile-base polymer powder and a not heat-treated acrylonitrile polymer powder to dissolve the mixture in an acidic solvent similarly has been proposed (see JP-B No. 62-57723), but a problem of impartation of anti-corrosion to the apparatus, and instability of a solution remained unsolved.
Further, a method of heat-treating a solution of polyacrylonitrile in dimethylformamide to convert polyacrylonitrile into a polymer with a cyclized structure has been proposed (see Polymer Science (USSR), 1968, Vol. 10, pp 1537-1542). However, in this proposal, since a polymer concentration is 0.5%, being a dilute solution, and a viscosity is too low, shaping and molding into a substantial fiber are difficult and, when one tries to increase the concentration, a polymer is precipitated, and use as a solution was impossible.
In addition, a solution obtained by modifying polyacrylonitrile with a primary amine has been proposed (see Journal of Polymer Science: Part A: Polymer Chemistry, 1990, Vol. 28, pp 1623-1636), but this solution imparts hydrophilicity to polyacrylonitrile itself, flame-resisting of which has not progressed, and technical idea is entirely different from a flame-resistant polymer-containing solution.
We succeeded in obtaining a dispersion containing a flame-resistant polymer which can be shaped into yarns or films, by reacting polyacrylonitrile using a nucleophile and an oxidizing agent in a polar solvent, and have already proposed this (see WO 2005/080448 A1).
As one means to further improve productivity of a flame-resistant product obtained by this method, improvement in stability in a step of producing a shaped body, particularly, production stability in a coagulating step including a coagulation site for shaping into a yarn shape, and a washing site for removing a chemical and a solvent remaining in a yarn is expected.
It could therefore be helpful to provide a dispersion containing a flame-resistant polymer which can improve shaping stability of the flame-resistant polymer during ejection from a die orifice, and physical stability of a shaped product in a washing step.